Radio frequency identification (RFID) systems typically include at least one reader and a plurality of transponders, which are commonly termed credentials, cards, tags, or the like. The transponder may be an active or passive radio frequency communication device, which is directly attached to or embedded in an article to be identified or otherwise characterized by the reader. Alternatively, the transponder may be embedded in a portable substrate, such as a card or tag, carried by a person or an article to be identified or otherwise characterized by the reader. An active transponder is powered up by its own internal power supply, such as a battery, which provides the operating power for the transponder circuitry. In contrast, a passive transponder is characterized as being dependent on the reader for its power. The reader “excites” or powers up the passive transponder by transmitting excitation signals of a given frequency into the space surrounding the reader, which are received by the transponder and provide the operating power for the circuitry of the recipient transponder.
Communication between the reader and transponder is enabled by cooperative resonant circuits which are provided in each reader and transponder. The resonant circuit of a reader includes an inductor, typically in the form of an antenna, which magnetically couples to the inductor in the resonant circuit of a compatible transponder through mutual inductance. The resonant circuit of the transponder correspondingly includes an inductor which magnetically couples to the inductor in the resonant circuit of the reader through mutual inductance.
Communication is initiated when a transponder is proximally positioned relative to the reader. The reader has a power supply which conveys a current to the reader resonant circuit causing the reader antenna to produce an excitation signal in the form of an electromagnetic field. The excitation signal couples to the antenna of the proximally-positioned transponder through mutual inductance and the excitation signal powers and clocks the transponder circuitry initiating operation of the transponder.
Transponder operation comprises generation of a response signal at a specified frequency and transmission of the transponder response signal back to the reader. In particular, the transponder resonant circuit receives a current in response to the excitation signal, which causes the transponder antenna to produce a response signal in the form of an electromagnetic field. The response signal couples to the reader antenna through mutual inductance in substantially the same manner as described above with respect to coupling of the excitation signal to the transponder antenna.
The transponder typically employs frequency or amplitude modulation of the response signal to encode data stored in the memory of the transponder circuitry into the response signal. When the response signal couples to the reader antenna, a corresponding current is induced in the reader antenna at the specified frequency. The reader processes the induced current to read the data encoded in the response signal. The resulting data may be communicated to an output device, such as an access control panel, access control locking device, parking gate, display, printer, or storage device, and simultaneously, or alternatively, communicated to a local host proximally located computer or remote host, if a host computer is networked into the RFID system.
An important operating parameter of the reader is the range of the reader when communicating with a transponder. The read range is strongly affected by the strength of the electromagnetic field generated by the reader resonant circuit. To generate a field strength which provides the reader with adequate range, the designer of the reader must properly specify a resonant circuit which is appropriately tuned to a predetermined frequency for the desired application of the RFID system.
Another important operating parameter of the reader is antenna impedance. It is desirable that the impedance of the antenna in the reader of an RFID system be specified to match the impedance of the antenna driver. However, the impedance of the reader antenna is often altered by the characteristics of the operating environment in which the reader resides. Additionally, the impedance of the reader antenna can be disturbed during the antenna or reader fabrication process resulting in a detuned resonant circuit. Further, the transponders may also have unique characteristics that vary based on the type of transponder, the manufacturer of the transponder, etc. or may vary from card to card even if the cards are of a same type.
It is generally possible to tune a reader. However, tuning a reader may be static operation (occur only once) and only account for a fixed and partial set of variables or conditions that affect the reader and/or transponder. Further, current tuning methods have sacrificed maximum potential read range due to singular radio frequency (RF) settings/performance used on all credential types and across all RF modulation schemes. The end result is compromised (e.g., subdued) RF performance across the credential range, which may result in reduced read range and potential RF holes (areas near the reader when a credential cannot be read). Also, current tuning methods do not account or optimize performance across credential types, modulation protocols, for different coupling situations (e.g., distance, mounting environment, coil-to-coil coupling, etc.) between the credential and reader, and other factors.